As a result of the growing integration density in the semiconductor industry (Moore's law), photolithographic masks have to project increasingly smaller structures onto wafers. This trend towards growing integration densities is accounted for, among other things, by shifting the exposure wavelength of the lithography devices to smaller and smaller wavelengths. Presently, in lithography devices an ArF (argon fluoride) excimer laser is often used as a light source, emitting at a wavelength of approximately 193 nm.
At present, lithography systems are being developed which use electromagnetic radiation in the EUV (extreme ultraviolet) wavelength range (in the range of 10 nm to 15 nm). These EUV lithography systems are based on a completely new beamline concept, which preferably uses reflective optical elements including reflective photomasks.
Due to the small wavelength in the EUV range, photomasks have to fulfill extreme requirements with regard to the precision of predetermined surfaces. Deviations in the surface topology in the multilayer mirror systems of photolithographic EUV masks in the single-digit nanometer range already lead to a significant variation of the reflected intensity within the EUV beam. Due to the technological challenges for the manufacture of present day and in particular future photolithographic masks and the high costs implied by this, photomasks not fulfilling a predetermined specification are repaired whenever possible.
Typical photomasks can comprise many large regions with uniform, periodically repeating patterns. FIG. 1 schematically shows as a typical example a section of a photomask 100 with a periodic arrangement of absorbing tracks 110 as structural elements which are arranged on the substrate 120 of the mask 100 (“lines and spaces”). A unique correlation of two images of a section of a photomask with periodic patterns is difficult. Within the context of this application, the terms photolithographic mask, photomask and mask are used synonymously.
Certain defects on a photomask, in particular on EUV-masks, are not visible in an image of a scanning electron microscope, because they generate not enough topology contrast. In the image of an atomic force microscope, on the other hand, these defects show up, for example, as bulges or indentations having a height or depth in the single-digit nanometer range. Such defects may, for example, be repaired by an electron beam repair tool, for example the MeRiT® tool of the applicant. For this, it is necessary, however, to superimpose the images of the atomic force microscope (AFM) and the scanning electron microscope (SEM) with an accuracy in the single-digit nanometer range, in order to repair at the appropriate location. Due to the issue described above, this faces serious difficulties.
An AFM as well as a SEM may both be calibrated with marks that are present on every photomask in a marginal region and between the dies, such that these devices work with absolute mask coordinates. However, the individual markers are spaced apart from one another on the mask by such a distance that, in general, they are not shown in an image or image section generated by a SEM or AFM. The accuracy of the superposition of two images or image sections having been recorded with an AFM and a SEM is then limited by the calibration and the accuracy of the movements of the microscope stage of the repair tool. The achievable accuracy is in general not sufficient in order to be able to reliably repair the defects described above with a SEM. With the help of the calibration by markers on the mask it may, however, be assured that the images or image sections of the mask at least partially overlap.
For the precision alignment of the two images, three methods exist at present: first, markers may be positioned “blindly” around the assumed defect location with a repair tool. These markers are then clearly visible in an image subsequently recorded with an AFM. For the repair, the two images having been generated from an AFM-scan and an SEM-scan can then be precisely superimposed. This method has the disadvantage that markers can potentially be positioned at the wrong locations and by this the photomask may be processed in a wrong location. Furthermore, the additional marking step costs further time.
Second, the WO 2013/010976 A2 describes a method for localizing the above described defects on the substrate of a photolithographic mask and for applying a marker in the vicinity of the defect, which uses three different measurement devices. Hence, the disclosed method is on the one hand labor-intensive and on the other hand it may be necessary to remove the applied marker(s) again at the end of the repair process in an additional process step.
Finally, a previous scan of a defect location with an SEM may leave behind what is called a “scanbox”, which is then visible in a subsequent scan of the defect location with an AFM in the corresponding image. A scanbox is created when an electron beam essentially carbonizes organic molecules absorbed on an imaged surface and therefore deposits a permanent product containing carbon. This method has the disadvantage that scanboxes are unwanted. In addition, they are only created if the SEM is contaminated with volatile organic compounds.